1. Field of the Invention
The present invention relates generally to concrete reinforcing, and in particular to a reinforcing dowel system and method for concrete slabs.
2. Description of the Prior Art
A typical method of constructing roadways and highways involves pouring a concrete slab over a suitable base course of crushed rock or other material. Concrete is often the preferred material for roadway slabs because it tends to be relatively strong, durable, and cost-effective over the life of a roadway or highway. Moreover, a well-designed and well-constructed concrete roadway may be less susceptible to potholes and the need for resurfacing than a comparable asphalt roadway. The interstate highway system and other major roadways are therefore constructed largely of concrete.
In order to maximize the useful life of a concrete roadway, the design must accommodate various loads to which the slab is subjected and the stresses resulting therefrom. For example, thermal loads result from variations in the ambient temperature. Annual temperature variations of more than 60 degrees Celsius are not uncommon in many areas. Expansion/contraction cycles from temperature changes can be accommodated somewhat by forming construction or expansion joints at regular intervals to allow relative movement between adjacent slab sections. Expansion joints usually comprise gaps between adjacent concrete slabs. The gaps are filled with resilient materials, such as an elastomeric caulk. The caulk expands and compresses in response to the thermal loads on the slabs. Concrete normally reaches its design strength approximately 28 days after it is poured. Partially-cured or "green" concrete can be saw cut relatively easily. Therefore, construction and expansion joints are often cut in concrete slabs when they are partially cured and the fresh concrete is green.
Special slip-forming equipment is commonly used for pouring continuous slabs for concrete roadways. Cracks are expected in large concrete slabs, such as roadways constructed with slip-forming equipment. The crack locations can be controlled by sawing concrete slabs part-way through to form construction joints. Weakened areas are thus formed with less cross-sectional area to resist bending and other stresses. By providing such joints at periodic intervals, cracks are generally limited to the areas of the construction joints whereby intermediate areas tend to remain intact. After partial saw cut construction joints have been formed, a slab will tend to crack along them. The slab will thus be effectively divided into multiple slab sections.
In conventional reinforced concrete roadway construction, steel reinforcing rod dowels are placed across the areas in which the construction joints are cut. The dowels transfer loads between the slab sections. Otherwise the slab sections could act as independent structural elements separated from each other by cracks along their respective construction joints. However, with the construction joints spanned by reinforcing dowels, the adjacent slab sections are generally retained in alignment with each other.
A common practice to accommodate expansion and contraction of the adjacent slab sections is to apply a coat of grease to one end of each reinforcing dowel to permit limited longitudinal slippage. The grease-coated end of a reinforcing dowel can thus reciprocate slightly within the concrete slab in which it is encased.
In conventional concrete slab construction, the dowels are typically secured in place by wire baskets while the concrete slab is being poured. The baskets are adapted for placement on a crushed rock roadway base. The dowels are thus supported over the roadway base at predetermined heights and can be placed approximately medially within the slab.
Such prior art dowel and basket combinations suffered from several disadvantages. For example, the relatively small diameters of the steel reinforcing bar dowels tended to concentrate shear forces over relatively small contact areas. The aforementioned technique of coating one end of a dowel with grease tended to introduce annular clearance between the slab and the dowel. This clearance could lead to the dowels working themselves even more loose after being subjected to relatively large dynamic loads over prolonged periods of time. Loose dowels can lead to premature slab deterioration and failure.
Moreover, previous basket designs included reinforcing wire members which extended across a construction joint. The normal procedure was to cut these joint-spanning wires before the cement was placed. Omitting this step could result in "locked" construction joints. Previous basket designs also interfered with concrete pouring operations.
Heretofore there has not been available a tubular reinforcing dowel system and method with the advantages and features of the present invention.